The primary goal is to advance understanding of the pathobiology and natural history of and to develop new strategies to prevent and treat graft-versus-host disease in pediatric patients. This is a collaborative project led by the NCI Experimental Transplantation Immunology Branch in collaboration with multiple NIH Intramural Investigators. The following projects/efforts are currentlyunderway or in preparation: 1) Natural History Trial of GVHD; 2) Extracorporeal Photophersis; 3) National Working Groups to Develop Diagnostic Standards and Supportive Care Guidelines; 4) Novel Therapeutics for GVHD